Organic azo pigments have been widely used in the paint industry based on range of color display and relative low cost. However, organic pigments in the form initially obtained after chemical synthesis, i.e. crude pigments, are generally unsuitable for immediate use as pigments. Most of the crude pigments have small particle size and are transparent in color, resulting in poor lightfastness. These pigments therefore often require additional treatment during preparation to modify particle size and particle shape and/or crystal structure to increase opacity and/or improve lightfastness. Such methods include heating the crude pigments in the presence of a solvent or in an aqueous medium at a higher temperature and, quite often, placing the crude pigment in a pressure reactor to achieve the desirable particle size.
U.S. Pat. No. 4,476,052 discloses a process for the preparation of azo pigment by heating the pigment obtained after coupling in an organic solvent at 80–180° C. The addition of such organic solvent improves opacity for the pigment, however, the flammability of the solvent and its recovery not only increases the processing cost but also renders handling of the pigment inconvenient and a potential safety hazard in most traditional azo plants.
Thus, there exists a need for an efficient and effective process to prepare an azo pigment whereby the azo pigment has strong lightfastness, large particle size, and opacity without requiring a flammable solvent component.